1. Field of the Invention
The present invention relates to an improved seamless endless belt having excellent heat resistance and dimensional stability and to a process for manufacturing same. More particularly, the present invention relates to a seamless endless belt utilizable in high temperature applications, for example, a conveyer belt utilizable for conveying sheets of paper for ink-jet printers and a belt for image-fixing in printers and facsimile machines, which requires high precision in dimensional size, contraction-resistance, mechanical strength and heat resistance as well as a process for manufacturing same involving the step of simultaneous inflation-and-biaxially stretching of a polyether-ether-ketone resin.
2. Description of the Prior Art
An endless belt made of a synthetic resin has excellent mechanical strength per unit thickness as compared with a conventional endless belt made of rubber. Accordingly, the former belt is advantageous in that the thickness of the belt can be reduced so as to make the device lightweight and to be used at a relatively high temperature by virtue of reduced heat accumulation. Furthermore, the former belt is superior in processability so that it may be blended with various resins possessing favorable characteristics to manufacture a belt of desired properties. These beneficial properties make the former belt useful for various applications where the device is desired to be smaller or lightweight. In general, an endless belt of a synthetic resin is manufactured by bonding both end portions of a strip of a synthetic resin film by the aid of a binder or by fusing the end portions together and then cooling to effect bonding. However, the endless belt thus formed has a step at the seam where both ends of the resin film overlap, so that the movement of the belt becomes unstable at the seam, or otherwise, the belt tends to be damaged or broken at the seam with lapse of time.
In order to overcome the above problems, a seamless endless belt was proposed which is manufactured by cutting a tube of a synthetic resin in round slices perpendicular to the longitudinal direction of the tube. The belt of this type is seamless so that care is unnecessary for controlling the position of the seam and moving parts can thus be simplified. Further, there is an additional advantage that the life of the belt is prolonged and the movement of the belt becomes smooth. Accordingly, such a seamless endless belt finds wide industrial applications as a conveyer or power transmission belt.
The seamless endless belts are thus advantageous in a wider variety of applications as compared with ordinary belts having a seam, but require various characteristics depending on the intended use. Especially, the following characteristics are required for general applications: low contraction and elongation at high temperatures, good dimensional stability in particular at high temperatures, no fluctuation in thickness, and high mechanical strength, especially tensile strength and Young's modulus for withstanding heavy load during continuous use. These characteristics depend greatly on the physical and mechanical properties of the starting synthetic resin and also on the processing conditions for manufacturing the tube from the resin. In case such a seamless endless belt is to be utilized for electronic apparatus, it is especially required that the belt have excellent precision of size, in particular, diameter. This precision in size of the diameter is necessary not only at the time of manufacturing the belt but also after the continuous use of the belt for a long period of time. Accordingly, the belt should have an extremely low rate of thermal contraction especially at a high temperature, a high elasticity over a wide range of temperature to prevent change in size from elongation caused by high tension, and a precision in thickness devoid of any fluctuation.
When such a seamless endless belt is used, for example, as a conveyer belt for conveying sheets of paper in an ink-jet printer or as a belt for image-fixing in a printer or facsimile machine, the belt is exposed to high temperature since the ink-jet printer uses a molten hot-melt type ink and the fixing is carried out by using a toner at a temperature as high as 150.degree. C. or more. When the belt is used for the above mentioned applications, it must be furnished with such a characteristic that no fluctuation in dimension occurs even at a high temperature above 200.degree. C. and a certain degree of mechanical strength is exhibited at such high temperatures.
Among synthetic resins, polyester resin has recently become regarded as having strong mechanical properties and good processability. In addition, the so-called biaxially stretching technique is known to increase the mechanical strength of a film. In view of the above circumstances, a seamless endless belt shaped from a biaxially stretched tube of a thermoplastic polyester resin was proposed for satisfying the above mentioned requirements to a certain degree. As the polyester resin generally has a glass transition temperature lower than 100.degree. C., the use of a seamless endless belt manufactured from the polyester resin at temperatures exceeding 150.degree. C. significantly deteriorates the tensile strength and Young's modulus so that the belt is elongated by load or contracted on account of residual strain at the time of shaping. For these reasons, there is a great demand for developing a seamless endless belt which is extremely high in dimensional stability and is utilizable at a high temperature.
In general, seamless endless tubes are manufactured by cutting a tube in round slices perpendicular to the longitudinal direction of the tube, irrespective of what resin is employed as a material. In order to obtain seamless endless belts excellent in mechanical strength and running stability, the starting seamless tube should have a thickness of at least 50 .mu.m. For obtaining a tube having a thickness of 50 .mu.m or more and improved in tensile strength, breakdown elongation, Young's modulus and stiffness by the biaxial stretching treatment, however, the unstretched starting tube to be subjected to the treatment must have a thickness of 3-16 times as much as the resultant stretched tube. It is extremely difficult to manufacture unstretched tubes excellent in precision of thickness satisfying the above criterion. Furthermore, in case too thick an unstretched tube is stretched according to the inflation method, there arises the problem that a significantly high stress is needed and fluctuation tends to occur in thickness.
In view of the foregoing, it is extremely difficult to provide a seamless endless belt utilizable for electronic apparatus having high heat-resistance, dimensional stability, mechanical strength and moving stability as well as a process for manufacturing such tube. Accordingly, there is a great demand in this art for developing such seamless endless tubes and a new process for manufacturing same.